Conventional scanning electron microscopes are large immobile devices. Although scanning electron microscopes have many applications, such as semiconductor related inspection and testing, conventional scanning electron microscopes are limited in their usefulness because of their size, immobility, and associated costs. For instance, because the sample being observed, as opposed to the electron microscope, must be moved during the inspection process, a conventional scanning electron microscope requires the use of a vacuum chamber that is much larger than the sample. Further, the sample must be positioned at an angle relative to a conventional scanning electron microscope to produce a beam incidence angle required for three-dimensional-like surface feature imaging, which makes handling large or delicate samples difficult. Moreover, throughput of a conventional electron microscope is limited because only one electron microscope can observe a sample at a time.
An effort to improve electron-beam systems has resulted in miniature electron-beam microcolumns ("microcolumns"). Microcolumns are based on microfabricated electron "optical" components and field emission sources operating under principles similar to scanning tunneling microscope ("STM") aided alignment principles. The alignment principles used by microcolumns are similar to STMs in that a precision XY-Z positioner is used to control a sharp tip, in the case of a microcolumn a field emission tip, and to utilize the emission from the tip to measure the position of the tip. Microcolums are discussed in general in the publication "Electron-Beam Microcolumns for Lithography and Related Applications," by T. H. P. Chang et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3774-81, November/December 1996, which is incorporated herein by reference.
A conventional box-type microcolumn 10, as shown in FIG. 1, is positioned over a sample 20 with a support arm 22. Microcolumn 10 includes a positioner housing structure 30 upon which is mounted a field emitter source 40, such as a Schottky field emitter or a cold-field emitter. Microcolumn 10 also includes a support structure 32 and base plate structure 34, which support an electron "optical" column 50 and a detector assembly 60. Electron optical column 50 consists of lenses and deflectors to form a focused beam of electrons which can be scanned over the sample surface. Microcolumn 10 typically produces a 1 keV beam with a working distance in the range of 1-5 mm (millimeters). A short working distance provides high resolution microscopy, for example down to a 10 nm (nanometer) beam diameter or less, while a longer working distance may be used for applications requiring a larger field size, for example up to 150 .mu.m or larger. As shown in FIG. 1, the electron beam 42 produced by microcolumn 10 has a normal incidence with respect to the surface of sample 20.
FIG. 2 is a perspective view of box-type microcolumn 10 above sample 20. A typical example of a present day box-type microcolumn 10 is approximately 20.times.20 mm in the A and B dimensions and 22.5 mm in the C dimension as shown in FIG. 2.
FIG. 3 illustrates an exploded diagram of field emitter source 40 and electron optical column 50. Field emitter source 40 includes a field emitter tip 42, which may be a Zr/O/W Schottky field emitter tip or a cold-field emitter tip, such as a single crystal tungsten, hafnium carbide or diamond tip. Field emitter tip 42 is mounted on a three axis STM-like X-Y-Z positioner 44, which is contained in the positioner housing structure 30 shown in FIGS. 1 and 2. The X-Y-Z positioner 44 has a range of movement in the order of tens of micrometers to approximately 1 mm in the X, Y, and Z axes with nanometer-scale positioning capability and is used to align field emitter tip 42 with electron optical column 50. Typical present day dimensions of X-Y-Z positioner 44 are approximately 20.times.20.times.14 mm, which places a limitation on the dimensions of positioner housing structure 30.
The typical components of present day electron optical column 50 include a microsource lens 52 with an extractor and an anode with apertures of approximately a few micrometers and 100 .mu.m in diameter, respectively. Microsource lens 52 is followed by a beam limiting aperture 54, approximately a few micrometers in diameter, that is tailored to achieve optimum beam performance. The extractor and anode of microsource lens 52 and beam limiting aperture 54 are silicon electrodes bonded together using Pyrex insulating spacers (not shown) that are approximately 100-500 .mu.m thick. A double octopole deflection system 56 follows beam limiting aperture 54. Electron optical column 50 also includes an Einzel electron lens 58 that consists of three silicon electrodes with apertures of approximately 200 .mu.m in diameter and which are separated by Pyrex insulating spacers (not shown) that are approximately 250 .mu.m thick. The column length between field emitter tip 42 and the last electrode of Einzel lens 58 is approximately 3.5 mm.
Between Einzel lens 58 and the sample 20 is detector assembly 60. Detector assembly 60 may be a microchannel plate (MCP) based secondary/backscattered electron detector or a metal-semiconductor-metal (MSM) detector. Conventional Everhart-Thornley detectors are not used in conjunction with microcolumn 10 because of the difficulty in extracting secondary electrons to a Everhart-Thornley detector with current microcolumn design.
It is understood that FIG. 3 illustrates merely one example of many possible field emission sources and electron optical columns that may be used in microcolumn 10. For additional field emission sources and electron optical columns that may be used in microcolumn 10 and for information relating to the workings of microcolumn 10 in general, see the following articles and patents: "Experimental Evaluation of a 20.times.20 mm Footprint Microcolumn," by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, November/December 1996; "Electron Beam Technology--SEM to Microcolumn," by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; "Electron Beam Microcolumn Technology And Applications," by T. H. P. Chang et al., Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp. 4-12, 1995; "Lens and Deflector Design for Microcolumns," by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; "Miniature Schottky Electron Source," by H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.
As shown in FIG. 1, box-type microcolumn 10 produces a normal incidence electron beam that is useful primarily in applications such as lithography. However, as is well understood by those skilled in the art, for a general purpose scanning electron microscope it is important to be able to view a sample at an angle to obtain three-dimensional-like surface feature images. The 20.times.20 mm footprint of box-type microcolumn 10 along with its relatively short working distance (15 mm) limits the angle from which microcolumn 10 may view a sample. Consequently, the usefulness of box-type microcolumn 10 as a general purpose scanning electron microscope is limited.